1. Field of the Invention
The present invention relates to a light emitting diode, and more particularly, to a light emitting diode having an active region of a multi quantum well structure.
2. Description of the Related Art
In general, since Group III element nitrides, such as GaN, AlN and InGaN, have excellent thermal stability and a direct transition type energy band structure, they have recently come into the spotlight as materials for light emitting diodes (LEDs) in blue and ultraviolet regions. Particularly, an InGaN compound semiconductor has been considerably noticed due to its narrow band gap. LEDs using such a GaN-based compound semiconductor are used in various applications such as large sized full color flat panel displays, backlight sources, traffic lights, indoor illumination, high density light sources, high resolution output systems and optical communications.
FIGS. 1 and 2 are a sectional view and a schematic band diagram illustrating a conventional LED having an active region of a multi quantum well structure. FIG. 2(a) simply shows a band diagram of respective layers, and FIG. 2(b) shows a band diagram in an equilibrium state. FIG. 3 is a schematic band diagram when a forward voltage is applied to the LED of FIG. 2.
Referring to FIGS. 1 and 2, an LED comprises an N-type semiconductor layer 17, a P-type semiconductor layer 23 and an active region 19 interposed between the N-type and P-type semiconductor layers 17 and 23. In order to increase recombination efficiency of electrons and holes, a blocking layer 21 having a relatively wide bandgap may be interposed between the P-type semiconductor layer 23 and the active region 19.
The N-type and P-type semiconductor layers comprise Group III element nitride semiconductor layers, e.g., GaN semiconductor layers. The active region 19 is generally formed to have a multi quantum well structure in which well layers 19a and barrier layers 19b are alternately laminated. In an InGaN LED, an active region of a multi quantum well structure is generally formed by alternately laminating InGaN well layers 19a and GaN barrier layers 19b. The well layer 19a comprises a semiconductor layer with a smaller band gap than the N-type and P-type semiconductor layers 17 and 19 and the barrier layer 19b, thereby providing quantum wells in which electrons and holes are recombined.
Here, since polarization is generated by a piezoelectric field in an active region formed of GaN-based semiconductor layers, bands of the well layers 19a are inclined in a direction different from those of the barrier layers 19b, considering polarization in FIG. 2(b). The polarization generated by the piezoelectric field is generally known as the quantum confined stark effect (QCSE). The recombination rate of electrons and holes is decreased due to the QCSE, thereby decreasing luminous efficiency.
Referring to FIG. 3, when a forward voltage VF is applied to the LED, bands of the N-type semiconductor layer 17 are moved up. When a forward voltage which is similar to or higher than the bandgap potential of the P-type semiconductor layer 23 is applied to the LED, the conduction band Ec of the N-type semiconductor layer 17 is positioned higher than that of the P-type semiconductor layer 23. At this time, the closer to the N-type semiconductor layer 17 the barrier layer 19b is positioned in the active region, the higher the conduction band of the barrier layer is positioned, as shown in this figure. The arrangement of the bands of barrier layers 19b provides driving force for allowing carriers injected from the N-type semiconductor layer 17 not to be subjected to recombination in the active region 19 but to flow into the P-type semiconductor layer, which accordingly causes carrier overflow to occur as shown by a dotted line arrow. The excessive occurrence of such carrier overflow causes the recombination rate of electrons and holes to be deteriorated, thereby decreasing luminous efficiency.
Meanwhile, the blocking layer 21 is employed in order to decrease carrier overflow. The blocking layer 21 is formed of a semiconductor having a wider bandgap to prevent carrier overflow. However, the blocking layer 21 formed of the semiconductor having a wider bandgap has increased lattice mismatch with the P-type semiconductor layer 23, and thus, the crystalline quality of the P-type semiconductor layer 23 grown on the blocking layer 21 is deteriorated. Therefore, there is a certain limit to preventing carrier overflow using the blocking layer 21.
As applications of LEDs are extended to various fields including general illumination, the driving voltage applied to an LED is not limited to a conventional forward voltage of 3V or so but is continuously increased. The increase of the driving voltage causes carrier overflow to be more increased, and accordingly, thereby more decreasing the luminous efficiency of the LED. Therefore, in an LED operated under high voltage (or high current), it is required to develop a new technique for preventing carrier overflow besides the blocking layer 21 and to develop a technique for lowering the driving voltage of the LED.
Meanwhile, the conventional LED comprises an active region of a multi quantum well structure having the barrier layers 19b with the generally same thickness. The thickness of the barrier layers 19b is selected to promote stability of a process and to have optimum luminous characteristics under certain current conditions.
However, the LED may be operated under various current conditions in some cases. For example, in case of an AC LED driven under AC power source, the LED may be driven by AC current that varies continuously. In this case, it is difficult for the conventional LED having the barrier layers 19b with the same thickness to exhibit optimum luminous characteristics under both low and high current conditions.